The fluid catalyst cracking process (hereinafter FCC) has been extensively relied upon for the conversion of starting materials, such as vacuum gas oils, and other relatively heavy oils, into lighter and more valuable products. FCC involves the contact in a reaction zone of the starting material, whether it be vacuum gas oil or another oil, with a finely divided, or particulated, solid, catalytic material which behaves as a fluid when mixed with a gas or vapor. This material possesses the ability to catalyze the cracking reaction, and in so acting it is surface-deposited with coke, a by-product of the cracking reaction. Coke is comprised of hydrogen, carbon and other material such as sulfur, and it interferes with the catalytic activity of FCC catalysts. Facilities for the removal of coke from FCC catalyst, so-called regeneration facilities or regenerators, are ordinarily provided within an FCC unit. Coke-contaminated catalyst enters the regenerator and is contacted with an oxygen containing gas at conditions such that the coke is oxidized and a considerable amount of heat is released. A portion of this heat escapes the regenerator with the flue gas, comprised of excess regeneration gas and the gaseous products of coke oxidation. The balance of the heat leaves the regenerator with the regenerated, or relatively coke free, catalyst. The high heat evolved in the regeneration process creates high temperatures in the regenerator. In order to withstand the high temperatures the large regeneration vessel has an internal concrete-like refractory lining that insulates the metal shell from the high regenerator temperatures and erosion of the abrasive catalyst.
The fluidized catalyst is continuously circulated from the reaction zone to the regeneration zone and then again to the reaction zone. The fluid catalyst, as well as providing catalytic action, acts as a vehicle for the transfer of heat from zone to zone. Catalyst exiting the reaction zone is spoken of as being "spent", that is partially deactivated by the deposition of coke upon the catalyst. Catalyst from which coke has been substantially removed is spoken of as "regenerated catalyst".
The rate of conversion of the feedstock within the reaction zone is controlled by regulation of the temperature, activity of catalyst and quantity of catalyst (i.e. catalyst to oil ratio) therein. The most common method of regulating the reaction temperature is by regulating the rate of circulation of catalyst from the regeneration zone to the reaction zone which simultaneously increases the catalyst/oil ratio. That is to say, if it is desired to increase the conversion rate, an increase in the rate of flow of circulating fluid catalyst from the regenerator to the reactor is effected. Inasmuch as the temperature within the regeneration zone under normal operations is considerably higher than the temperature within the reaction zone, this increase in influx of catalyst from the hotter regeneration zone to the cooler reaction zone effects an increase in reaction zone temperature.
An increasing number of FCC units use an external catalyst cooler to provide additional flexibility in operation. The term external catalyst cooler generally refers to a shell and tube heat exchanger that circulates catalyst from the regenerator on the shell side of the exchanger and saturated steam or water on the tube side of the exchanger. Indirect heat exchange with the steam or water cools the catalyst that circulates through the cooler and provides a source of relatively lower temperature catalyst for recirculation to the regenerator or return to the FCC reaction zone. By lowering the temperature of the catalyst, independent of the coke combustion, the cooler allows the FCC unit to fully combust coke without excessive temperatures when processing heavier feedstocks that produce more coke or to control the catalyst circulation rate independent of the riser temperature.
Locating the heat exchanger tubes outside of the regenerator in an external cooler permits isolation from a majority of the catalyst inventory in the event of a tube rupture or other operational problems. The external location of the cooler relative to the regenerator requires a circulation of catalyst between the cooler and the regenerator. Normally the circulating catalyst enters or exits the cooler from the a large open volume of the regenerator. Air nozzles or aeration piping keep the catalyst in a fluidized state so that it can circulate through the cooler. The cooler can operate in a flow through mode where hot catalyst enters one end of the cooler and leaves through from an opposite end of the cooler or in a backmix mode where the catalyst enters and leaves through the opening without any net flow. Aeration is particularly important in the backmix mode where a high degree of turbulence is needed to obtain the necessary interchange of catalyst through the cooler.
In some cases the aeration air has caused failure of the catalyst coolers by the rupture of the heat exchange tubes. It has now been found that the passage or accumulation of debris in the catalyst cooler led to the failure of the heat exchange tubes. Along with the fine particles of FCC catalyst that flow through an FCC unit, a small amount of debris also moves through the FCC unit. This debris normally consists of pieces of spalled or broken refractory lining from the inside of the vessel or agglomerated masses of the fine catalyst particles. This debris seldom causes any problem in most FCC units, but passes downwardly through the vessels and piping and accumulates in inactive areas of the unit for removal during normal maintenance. However, it was discovered that when such debris enters the catalyst cooler it can disrupt the airflow from the aeration nozzles or air distribution pipes. Surprisingly this disruption of air flow from accumulated debris has been found to cause tube failures at locations remote from the debris and the aeration nozzle outlets.